Security element comprising micro- and macrostructures

ABSTRACT

A security element which is difficult to copy includes a layer composite which has microscopically fine, optically effective structures of a surface pattern, which are embedded between two layers of the layer composite. In a plane of the surface pattern, which is defined by co-ordinate axes x and y, the optically effective structures are shaped into an interface between the layers in surface portions of a holographically non-copyable security feature. In at least one surface portion the optically effective structure is a diffraction structure formed by additive superimposition of a macroscopic superimposition function (M) with a microscopically fine relief profile (R). Both the relief profile (R), the superimposition function (M) and also the diffraction structure are functions of the co-ordinates x and y. The relief profile (R) is a light-diffractive or light-scattering optically effective structure and, following the superimposition function (M), retains the predetermined profile height. The superimposition function (M) is at least portion-wise steady and is not a periodic triangular or rectangular function. In comparison with the relief profile (R) the superimposition function (M) changes slowly. Upon tilting and rotation of the layer composite the observer sees on the illuminated surface portions light, continuously moving strips which are dependent on the viewing direction.

The invention relates to a security element as set forth in theclassifying portion of claim 1.

Such security elements comprise a thin layer composite of plasticmaterial, wherein at least relief structures from the group consistingof diffraction structures, light-scattering structures and flat mirrorsurfaces are embedded into the layer composite. The security elementswhich are cut out of the thin layer composite are stuck on to articlesfor verifying the authenticity of the articles.

The structure of the thin layer composite and the materials which can beused for same are described for example in U.S. Pat. No. 4,856,857. Itis also known from GB 2 129 739 A for the thin layer composite to beapplied to the article by means of a carrier film.

An arrangement of the kind set forth in the opening part of thisspecification is known from EP 0 429 782 B1. The security element whichis stuck on to a document has an optically variable surface patternwhich is known for example from EP 0 105 099 and which comprises surfaceportions arranged mosaic-like with known diffraction structures. So thata forged document, for faking apparent authenticity, cannot be providedwithout clear traces with a counterfeited security element which hasbeen cut out of a genuine document or detached from a genuine document,security profiles are embossed into the security element and intoadjoining portions of the document. The genuine document differs byvirtue of the security profiles which extend seamlessly from thesecurity element into adjoining portions of the document. The operationof embossing the security profiles interferes with recognition of theoptically variable surface pattern. In particular the position of theembossing punch on the security element varies from one example of thedocument to another.

It is also known for the security elements to be provided with featureswhich make it difficult or even impossible to counterfeit or copy usingconventional holographic means. For example EP 0 360 969 A1 and WO99/38038 describe arrangements of asymmetrical optical gratings. There,the surface elements have gratings which, used at different azimuthangles, form a pattern which is modulated in respect of brightness, inthe surface pattern of the security element. The pattern which ismodulated in respect of brightness is not reproduced in a holographiccopy. If, as described in WO 98/26373, the structures of the gratingsare smaller than the wavelength of the light used for the copyingoperation, such submicroscopic structures are no longer detected and arethus not reproduced in the copy in the same manner.

The protection arrangement to afford protection against holographiccopying described in EP 0 360 969 A1, WO 98/26373 and WO 99/38038 whichare referred to by way of example is achieved at the cost ofdifficulties in terms of production engineering.

The object of the invention is to provide an inexpensive novel securityelement which is to have a high level of resistance to attempts atforgery, for example by means of a holographic copying process.

That object is attained by a security element comprising a layercomposite with microscopically fine optically effective structures of asurface pattern, which are embedded between layers of the layercomposite, wherein the optically effective structures are shaped into areflecting interface between the layers in surface portions of asecurity feature in a plane of the surface pattern defined byco-ordinate axes and at least one surface portion of dimensions greaterthan 0.4 mm has a diffraction structure formed by additive orsubtractive superimposition of a superimposition function describing amacroscopic structure with a microscopically fine relief profile,wherein the superimposition function, the relief profile and thediffraction structure are a function of the co-ordinates and the reliefprofile describes a light-diffracting or light-scattering opticallyeffective structure which following the superimposition function retainsthe predetermined relief profile and the at least portion-wise steadysuperimposition function is curved at least in partial regions, it isnot a periodic triangular or rectangular function and it changes slowlyin comparison with the relief profile.

Advantageous configurations of the invention are set forth in theappendant claims.

Embodiments of the invention are described in greater detail hereinafterand illustrated in the drawing in which:

FIG. 1 is a cross-sectional view of a security element,

FIG. 2 shows a plan view of the security element,

FIG. 3 shows reflection and diffraction at a grating,

FIG. 4 shows illumination and observation of the security element,

FIG. 5 shows reflection and diffraction at a diffraction structure,

FIG. 6 shows the security feature at various tilt angles,

FIG. 7 shows a superimposition function and the diffraction structure incross-section,

FIG. 8 shows orientation of the security element by means ofidentification marks,

FIG. 9 shows a local angle of inclination of the superimpositionfunction,

FIG. 10 shows orientation of the security element by means of colorcontrast in the security feature,

FIG. 11 shows the diffraction structure with a symmetricalsuperimposition function,

FIG. 12 shows the security feature with color change, and

FIG. 13 shows an asymmetrical superimposition function.

Referring to FIG. 1, reference 1 denotes a layer composite, 2 a securityelement, 3 a substrate, 4 a cover layer, 5 a shaping layer, 6 aprotective layer, 7 an adhesive layer, 8 a reflecting interface, 9 anoptically effective structure and 10 a transparent location in thereflecting interface 8. The layer composite 1 comprises a plurality oflayer portions of various plastic layers which are applied successivelyto a carrier film (not shown here) and in the specified sequencetypically comprises the cover layer 4, the shaping layer 5, theprotective layer 6 and the adhesive layer 7. The cover layer 4 and theshaping layer 5 are transparent in relation to incident light 11. If theprotective layer 6 and the adhesive layer 7 are also transparent,indicia (not shown here) which are applied to the surface of thesubstrate 3 can be perceived through the transparent location 10. In anembodiment the cover layer 4 itself serves as a carrier film while inanother embodiment a carrier film serves for applying the thin layercomposite 1 to the substrate 3 and is thereafter removed from the layercomposite 1, as is described for example in above-mentioned GB 2 129 739A.

The common contact surface between the shaping layer 5 and theprotective layer 6 is the interface 8. The optically effectivestructures 9 are shaped into the shaping layer 5 with a structure heightH_(St) of an optically variable pattern. As the protective layer 6 fillsthe valleys of the optically effective structures 9, the interface 8 isof the same shape as the optically effective structures 9. In order toachieve a high level of effectiveness in respect of the opticallyeffective structures 9 the interface 8 is provided with a metal coating,preferably comprising the elements from Table 5 of above-mentioned U.S.Pat. No. 4,856,857, in particular aluminum, silver, gold, copper,chromium, tantalum and so forth which as a reflection layer separatesthe shaping layer 5 and the protective layer 6. The electricalconductivity of the metal coating affords a high level of reflectioncapability in relation to visible incident light 11 at the interface 8.However, instead of the metal coating, one or more layers of one of theknown transparent inorganic dielectrics which are listed for example inTables 1 and 4 of above-mentioned U.S. Pat. No. 4,856,857 are alsosuitable, or the reflection layer has a multi-layer interference layersuch as for example a double-layer metal-dielectric combination or ametal-dielectric-metal combination. In an embodiment the reflectionlayer is structured, that is to say it covers the interface 8 onlypartially and in predetermined zones of the interface 8.

The layer composite 1 is produced as a plastic laminate in the form of along film web with a plurality of mutually juxtaposed copies of theoptically variable pattern. The security elements 2 are for example cutout of the film web and joined to a substrate 3 by means of the adhesivelayer 7. The substrate 3 which is mostly in the form of a document, abanknote, a bank card, a pass or identity card or another important orvaluable article is provided with the security element 2 in order toverify the authenticity of the article.

FIG. 2 shows a portion of the substrate 3 with the security element 2. Asurface pattern 12 is visible through the cover layer 4 (FIG. 1) and theshaping layer 5 (FIG. 1). The surface pattern 12 is disposed in a planedefined by the co-ordinate axes x, y and includes a security feature 16comprising at least one surface portion 13, 14, 15 which is clearlyvisible in the contour thereof with the naked eye, that is to say thedimensions of the surface portion are greater than 0.4 mm at least inone direction. The security feature 16 is shown with double framinglines in FIG. 2, for reasons relating to the drawing. In anotherembodiment the security feature 16 is surrounded by a mosaic consistingof surface elements 17 through 19 of the mosaic described inabove-mentioned EP 0 105 099 A1. In the surface portions 13 through 15and optionally in the surface elements 17 through 19 the opticallyeffective structures 9 (FIG. 1) such as microscopically fine diffractivegratings, microscopically fine, light-scattering relief structures orflat mirror surfaces are shaped into the interface 8 (FIG. 1).

Reference is made to FIG. 3 to describe how the light 11 which isincident on the interface 8 (FIG. 1) is reflected by the opticallyeffective structure 9 and deflected in a predetermined manner. Theincident light 11 is incident on the optically effective structure 9 inthe layer composite 1 in the diffraction plane 20 which is perpendicularto the surface of the layer composite 1 with the security element 2(FIG. 1) and which includes a surface normal 21. The incident light 11is a parallel bundle of light beams and includes the angle of incidenceα with the surface normal 21. If the optically effective structure 9 isa flat mirror surface in parallel relationship with the surface of thelayer composite 1 the surface normal 21 and the direction of thereflected light 22 form the sides of the reflection angle β, whereinβ=−α. If the optically effective structure 9 is one of the knowngratings, the grating deflects the incident light 11 into variousdiffraction orders 23 through 25 determined by the spatial frequency fof the grating, in which respect it is assumed that the grating vectordescribing the grating is in the diffraction plane 20. The wavelengths λcontained in the incident light 11 are deflected into the variousdiffraction orders 23 through 25 at the predetermined angles. Forexample the grating deflects violet light (λ=380 nm) simultaneously asbeam 26 into the plus 1st diffraction order 23, as beam 27 into theminus 1st diffraction order 24 and as beam 28 into the minus 2nddiffraction order 25. Light components of longer wavelengths λ of theincident light 11 will issue in directions involving larger diffractionangles relative to the surface normal 21, for example red light (λ=700nm) into the directions identified by the arrows 29, 30, 31. Thepolychromatic incident light 11, as a consequence of diffraction at thegrating, is fanned out into the light beams of the various wavelengths λof the incident light 11, that is to say the visible part of thespectrum extends in the range between the violet light beam (arrow 26 or27 or 28 respectively) and the red light beam (arrow 29 or 30 or 31respectively) in each diffraction order 23 or 24 or 25 respectively. Thelight diffracted into the zero diffraction order is the light 22 whichis reflected at the reflection angle β.

FIG. 4 shows a diffraction grating 32 which is shaped in the surfaceelements 17 (FIG. 2) through 19 (FIG. 2) and whose microscopically finerelief profile R(x, y) has for example a sinusoidal, periodic profilecross-section of constant profile height h and with the spatialfrequency f. The averaged-out relief of the diffraction grating 32establishes a central plane or surface 33 which is arranged parallel tothe cover layer 4. The light 11 which is incident in parallelrelationship passes through the cover layer 4 and the shaping layer 5and is deflected at the optically effective structure 9 (FIG. 1) of thediffraction grating 32. The parallel diffracted light beams 34 of thewavelength λ leave the security element 2 in the direction of view of anobserver 35 who, when the surface pattern 12 (FIG. 2) is illuminatedwith the light 11 incident in parallel relationship, sees the coloredsurface elements 17, 18, 19 which shine brightly.

In FIG. 5 the diffraction plane 20 is in the plane of the drawing. Adiffraction structure S(x, y) is shaped in at least one of the surfaceportions 13 (FIG. 2) through 15 (FIG. 2) of the security feature 16(FIG. 2), the central surface 33 of the diffraction structure beingcurved or inclined locally relative to the surface of the layercomposite 1. The diffraction structure S(x, y) is a function of theco-ordinates x and y in the plane of the surface pattern 12 (FIG. 2),which is parallel to the surface of the layer composite 1 and in whichthe surface portions 13, 14 (FIG. 2), 15 lie. At each point P(x, y) thediffraction structure S(x, y) determines a spacing z relative to theplane of the surface pattern 12, which spacing is in parallelrelationship with the surface normal 21. Described in broader terms, thediffraction structure S(x, y) is the sum of the relief profile R(x, y)(FIG. 4) of the diffraction grating 32 (FIG. 4) and a clearly definedsuperimposition function M(x, y) of the central surface 33, wherein S(x,y)=R(x, y)+M(x, y). By way of example the relief profile R(x, y)produces the periodic diffraction grating 32 with the profile of one ofthe known sinusoidal, asymmetrically or symmetrically sawtooth-shaped orrectangular forms.

In another embodiment the microscopically fine relief profile R(x, y) ofthe diffraction structure S(x, y) is a matt structure instead of theperiodic diffraction grating 32. The matt structure is a microscopicallyfine, stochastic structure with a predetermined scatteringcharacteristic for the incident light 11, wherein with an anisotropicmatt structure instead of a grating vector, a preferred direction isinvolved. The matt structures scatter the perpendicularly incident lightinto a scattering cone with a spread angle which is predetermined by thescattering capability of the matt structure and with the direction ofthe reflected light 22 as the axis of the cone. The intensity of thescattered light is for example at the greatest on the axis of the coneand decreases with increasing distance in relation to the axis of thecone, in which respect the light which is deflected in the direction ofthe generatrices of the scattering cone is still just perceptible to anobserver. The cross-section of the scattering cone perpendicularly tothe axis of the cone is rotationally symmetrical, in the case of a mattstructure which is referred to here as ‘isotropic’. If in contrast thecross-section is upset in the preferred direction, that is to sayelliptically deformed, with the short major axis of the ellipse inparallel relationship with the preferred direction, the matt structureis referred to here as being ‘anisotropic’.

Because of the additive or subtractive superimposition the profileheight h (FIG. 4) of the relief profile R(x, y) is not changed in theregion of the superimposition function M(x, y), that is to say therelief profile R(x; y) follows the superimposition function M(x, y). Theclearly defined superimposition function M(x, y) can be at leastportion-wise differentiated and is curved at least in partial regions,that is to say ΔM(x, y)≠0, periodically or aperiodically, and is not aperiodic triangular or rectangular function. The periodicsuperimposition functions M(x, y) have a spatial frequency F of at most20 lines/mm. For good visibility, connecting sections between twoadjacent extreme values of the superimposition functions M(x, y) are atleast 0.025 mm long. The preferred values for the spatial frequency Fare limited to at most 10 lines/mm and the preferred values in respectof the spacing of adjacent extreme values are at least 0.05 mm. Thesuperimposition function M(x, y) thus varies as a macroscopic functionin the steady region slowly in comparison with the relief profile R(x,y).

A line 36 (FIG. 2) establishes a section line, projected on to the planeof the surface pattern 12 (FIG. 2), of the diffraction plane 20 with thecentral plane 33. The superimposition function M(x, y) has at any pointP(x, y) on the connecting sections parallel to the line 36, with steadyportions, a gradient 38, grad(M(x, y)). In general terms, the gradient38 means the component of the grad(M(x, y)) in the diffraction plane 20as the observer 35 establishes the optically effective diffraction plane20. At any point of the surface portion 13, 14, 15 the diffractiongrating 32 has an inclination γ which is predetermined by the gradient38 of the superimposition function M(x, y).

The deformation of the central surface 33 causes a new, advantageousoptical effect. That effect is explained on the basis of the diffractioncharacteristics at intersection points A, B, C of the surface normal 21and normals 21′, 21″ to the central surface 33, for example along theline 36. Refraction of the incident light 11, the reflected light 22 andthe diffracted light beams 34 at the interfaces of the layer composite 1is not shown for the sake of simplicity in FIG. 5 and is not taken intoaccount in the calculations hereinafter. At each intersection point A,B, C the inclination y is determined by the gradient 38. The normals 21′and 21″, the grating vector of the diffraction grating 32 (FIG. 4) and aviewing direction 39 of the observer 35 are disposed in the diffractionplane 20. The angle of incidence α (FIG. 3) which is included by thenormals 21, 21′, 21″ shown in broken line and the white light 11incident in parallel relationship changes in accordance with the angleof inclination γ. There is also a change therewith in the wavelength λof the diffracted light beams 34 which are deflected in a predeterminedviewing direction 39 to the observer 35. If the normal 21′ is inclinedaway from the viewer 35, the wavelength x of the diffracted light beams34 is greater than if the normal 21″ is inclined towards the observer35. In the example shown for illustration purposes, from the point ofview of the observer 35, the light beams 34 which are diffracted in theregion of the intersection point A are of a red color (λ=700 nm). Thelight beams 34 diffracted in the region of the intersection point B areof a yellow-green color (λ=550 nm) and the light beams 34 diffracted inthe region of the intersection point C are of a blue color (λ=400 nm).As in the illustrated example the inclination γ changes continuouslyover the curvature of the central surface 33, the entire visiblespectrum is visible for the observer 35 along the line 36 on the surfaceportion 13, 14, 15, the color bands of the spectrum extending on thesurface portion 13, 14, 15 in perpendicular relationship to the line 36.So that the color bands of the spectrum can be perceived by the observer35 at a 30 cm distance, at least 2 mm length or more is to be adoptedfor the distance between the intersection points A and C. Outside thevisible spectrum, the surface of the surface portion 13, 14, 15 is agray of low light intensity. When the layer composite 1 is tilted aboutthe tilt axis 41 perpendicularly to the plane of the drawing in FIG. 3,the angle of incidence α changes. The visible colour bands of thespectra are displaced in the region of the superimposition function M(x,y) continuously along the line 36. In the event of a tilting movement,for example in the clockwise direction about the tilt axis 41 of thelayer composite 1, the color of the diffracted light beam 34 at theintersection point A changes to yellow-green, the color of thediffracted light beam 34 at the intersection point B changes to blue andthe color of the diffracted light beam 34 at the intersection point Cchanges to violet. The variation in the colors of the diffracted light34 is perceived by the observer 35 as motion of the color bandscontinuously over the surface portion 13, 14, 15.

That consideration is applicable in respect of each diffraction order.How many color bands of how many diffraction orders are simultaneouslyseen by the observer on the surface portion 13, 14, 15 depends on thespatial frequency of the diffraction grating 32 and the number ofperiods and the amplitude of the superimposition function M(x, y) withinthe surface portion 13, 14, 15.

In another embodiment in which one of the matt structures is usedinstead of the diffraction grating 32, the observer 35, in the directionof the reflected light 22, sees only a light, white-gray band instead ofthe color bands. In the tilting movement, the light, white-gray bandmoves continuously like the color bands over the surface of the surfaceportion 13, 14, 15. In contrast to the color bands the light, white-grayband is visible to the observer 35, in dependence on the scatteringcapability of the matt structure, even when his viewing direction 39 isoblique relative to the diffraction plane 20. Hereinafter therefore theterm ‘strips 40’ (FIG. 6 a) is used to mean both the color bands of adiffraction order 23, 24, 25 and also the light, white-gray bandproduced by the matt structure.

Referring to FIG. 6 a, the displacement of the strip can be more easilyperceived by the observer 35 (FIG. 5) if there is a reference on thesecurity feature 16. Serving as the reference are identification marks37 (FIG. 2) arranged on the surface portion 13, 14, 15, for example onthe central surface portion 14, and/or a predetermined delimitationshape for the surface portion 13, 14, 15. Advantageously, the referenceestablishes a predetermined viewing condition which can be so adjustedby means of tilting movement of the layer composite 1 (FIG. 1) that thestrip 40 is positioned in predetermined relationship with respect to thereference. In the region of the identification marks 37 the opticallyeffective structure 9 (FIG. 1) of the interface 8 (FIG. 1) isadvantageously in the form of an optically effective structure 9, adiffractive structure, a mirror surface or a light-scattering reliefstructure which is shaped upon replication of the surface pattern 12 inregister relationship with the surface portions 13, 14, 15.Light-absorbent printing on the security feature 13 can however also beused as the reference for the movement of the strip 40 or theidentification mark 37 is produced by means of the structured reflectionlayer.

In a further embodiment of the security feature 16 as shown in FIG. 6the adjacent surface portions 13 and 15 which adjoin the central surfaceportion 14 on both sides serve as a mutual reference. The adjacentsurface portions 13 and 15 both have a diffraction structure S*(x, y).In contrast to the diffraction structure S*(x, y) the diffractionstructure S*(x, y) is the difference R−M of the relief function R(x, y)and the superimposition function M(x, y), that is to say S*(x, y)=R(x,y)−M(x, y). The color bands produced by the diffraction structure S*(x,y) are of a reversed color configuration with respect to the color bandsof the diffraction structure S(x, y), as is indicated in the drawing ofFIG. 6 a by means of a bold longitudinal edging for the strip 40. Forgood visibility of the optical effect without aids, the security feature16 is of a dimension of at least 5 mm and preferably more than 10 mmalong the co-ordinate axis y or the line 36. The dimensions along theco-ordinate axis x are more than 0.25 mm, but preferably at least 1 mm.

In the embodiment of the security feature 16 shown in FIGS. 6 a through6 c the oval surface portion 14 has the diffraction structure S(y) whichis dependent only on the co-ordinate y while the surface portions 13 and15 with the diffraction structure S*(y) which is dependent only on theco-ordinate y extend on both sides of the oval surface portion 14 alongthe co-ordinate y. The superimposition function is M(y)=0.5·y²·K,wherein K is the curvature of the central surface 33. The gradient 38(FIG. 5) and the grating vector of the diffraction grating 32 (FIG. 4)or the preferred direction of the ‘anisotropic’ matt structure areoriented in substantially parallel and anti-parallel relationshiprespectively with the direction of the co-ordinate y.

In general terms the azimuth φ of the grating vector or the preferreddirection of the matt structure is related to a gradient plane which isdetermined by the gradient 38 and the surface normal 21. The preferredvalues of the azimuth φ are 0° and 90°. In that respect, deviations inthe azimuth angle of the grating vector or of the preferred directionrespectively of δφ=±20° relative to the preferred value are admissiblein order in that region to view the grating vector or the preferreddirection respectively as substantially parallel or perpendicularrespectively to the gradient plane. In itself the azimuth φ is notrestricted to the specified preferred values.

The smaller the curvature K in each case is, the correspondingly higheris the speed of the movement of the strips 40 in the direction of thearrows (not referenced in FIGS. 6 a and 6 c) per unit of angle of therotational movement about the tilt axis 41. The strip 40 is shown asbeing narrow in FIGS. 6 a through 6 c in order clearly to illustrate themovement effect. The width of the strips 40 in the direction of thearrows which are not referenced is dependent on the diffractionstructure S(y). Particularly in the case of the color bands, thespectral color configuration extends over a major part of the surfaceportion 13, 14, 15 so that the movement of the strips 40 is to beobserved on the basis of travel of a portion in the visible spectrum,for example the color band red.

FIG. 6 b shows the security feature 16 after rotation about the tiltaxis 41 into a predetermined tilt angle at which the strips 40 of thetwo outer surface portions 13, 15 and the central surface portion 14 aredisposed on a line in parallel relationship to the tilt axis 41. Thatpredetermined tilt angle is determined by the choice of thesuperimposition function M(x, y). In an embodiment of the securityelement 2 (FIG. 2) a predetermined pattern is to be seen on the surfacepattern 12 (FIG. 2) only when in the security feature 16 the strip orstrips 40 assume a predetermined position, that is to say when theobserver 35 views the security element 2 under the viewing conditionsdetermined by the predetermined tilt angle.

In FIG. 6 c, after a further rotary movement about the tilt axis 41, thestrips 40 on the security feature 16 are moved away from each otheragain, as is indicated by the arrows (not referenced) in FIG. 6 c.

It will be appreciated that, in another embodiment, an adjacentarrangement of the central surface portion 14 and one of the two surfaceportions 13 and 15 is sufficient for the security feature 16.

FIG. 7 shows a cross-section taken along the line 36 (FIG. 2) throughthe layer composite 1, for example in the region of the surface portion14 (FIG. 2). So that the layer composite 1 does not become too thick andthus difficult to produce or use, the structure height H_(St) (FIG. 1)of the diffraction structure S(x; y) is restricted. The drawing which isnot true to scale in FIG. 7 illustrates by way of example thesuperimposition function M(y)=0.5·y²·K to the left of the co-ordinateaxis z on which the height of the layer composite expands, in section onits own. At any point P(x, y) of the surface portion 14 the value z=M(x,y) is limited to a predetermined variation value H=z₁−z₀. As soon as thesuperimposition function M(y) has reached the value z₁=M(Pj) for j=1, 2,. . . , n at one of the points P₁, P₂ . . . , P_(n), a discontinuitylocation occurs in the superimposition function M(y), and at thatdiscontinuity location, on the side remote from the point P₀, the valueof the superimposition function M(y) is respectively reduced by thevalue H to the height z₀, that is to say the value of thesuperimposition function M(x; y) used in the diffraction structure S(x;y) is the function value:z={M(x; y)+C(x; y)} modulo value H−C(x; y).

In that respect the function C(x; y) is limited in amount to a range ofvalues, for example to half the value of the structure height H_(St).The dislocation locations of the function {M(x; y)+C(x; y)} modulo valueH−C(x; y), which are produced for technical reasons, are not to becounted as extreme values in respect of the superimposition functionM(x; y). Equally, in given configurations, the values in respect of Hmay be locally smaller. In an embodiment of the diffraction structureS(x; y) the locally varying value H is determined by virtue of the factthat the spacing between two successive discontinuity locations P_(n)does not exceed a predetermined value from the range of between 40 μmand 300 μm.

In the surface portions 13 (FIG. 2), 14, 15 (FIG. 2) the diffractionstructure S(x, y) extends on both sides of the co-ordinate axis z andnot just, as is shown in FIG. 7, on the right of the co-ordinate axis z.Because of the superimposition effect the structure height H_(St) is thesum of the value H and the profile height h (FIG. 4) and equal to thevalue of the diffraction structure S(x, y) at the point P(x; y). Thestructure height H_(St) is advantageously less than 40 μm, preferredvalues in respect of the structure height H_(St) being <5 μm. The valueH of the superimposition function M(x, y) is restricted to less than 30μm and is preferably in the range of between H=0.5 μm and H=4 μm. On themicroscopic scale the matt structures have fine relief structuralelements which determine the scattering capability and which can only bedescribed with statistical parameters, such as for example meanroughness value R_(a), correlation length I_(c), and so forth, in whichrespect the values in respect of the mean roughness value R_(a) are inthe range of between 200 nm and 5 μm, with preferred values betweenR_(a)=150 nm and R_(a)=1.5 μm, while the correlation lengths I_(c), atleast in one direction, are in the range of between 300 nm and 300 μm,preferably between I_(c)=500 nm and I_(c)=100 μm. In the case of the‘isotropic’ matt structures the statistical parameters are independentof a preferred direction while in the case of the ‘anisotropic’ mattstructures relief elements are oriented with the correlation lengthI_(c) perpendicularly to the preferred direction. The profile height hof the diffraction grating 32 (FIG. 4) is of a value from the range ofbetween h=0.05 μm and h=5 μm, wherein the preferred values are in thenarrower range of h=0.6±0.5 μm. The spatial frequency f of thediffraction grating 32 is selected from the range of between f=300lines/mm and 3300 lines/mm. From about F=2400 lines/mm the diffractedlight 34 (FIG. 5) can still be observed only in the zero diffractionorder, that is to say in the direction of the reflected light 22 (FIG.5).

Further examples of the superimposition function M(x, y) are as follows:

M(x, y)=0.5·(x²+y²)·K, M(x, y)=a·{1+sin(2πF_(x)·x)·sin(2πF_(y)·y)}, M(x,y)=a·x^(1.5)+b·x, M(x, y)=a·{1+sin(2πF_(y)·y)}, wherein F_(x) and F_(y)are respectively the spatial frequency F of the superimposition functionM(x, y) in the direction of the co-ordinate axis x and y respectively.In another embodiment of the security feature 16 the superimpositionfunction M(x, y) is composed periodically from a predetermined portionof another function and has one or more periods along the line 36.

In FIG. 8 a the superimposition function M(x, y)=0.5·(x²+y²)·K, that isto say a portion of a sphere, and the relief structure R(x, y), that isto say an ‘isotropic’ matt structure, form the diffraction structureS(x, y) (FIG. 7) in the surface portion 14 which for example has acircular edging. The observer 35 (FIG. 5), in daylight, in accordancewith the viewing direction 39 (FIG. 5), sees a light, white-gray spot 42against a dark-gray background 43, the position of the spot 42 in thesurface portion 14 in relation to the identification mark 37 and thecontrast between the spot 42 and the background 43 being dependent onthe viewing direction 39. The extent of the spot 42 is determined by thescattering capability of the matt structure and the curvature of thesuperimposition function M(x, y). The security element 2 (FIG. 2) is tobe oriented to the predetermined viewing direction 39 for example bytilting about the tilt axis (41 (FIG. 5) and/or rotation about thesurface normal 21 (FIG. 5) of the layer composite 1 (FIG. 5) as in FIG.8 b in such a way that the spot 42 is within the identification mark 37which is arranged for example at the center of the surface portion 14with a circular edging.

FIG. 9 shows the light-diffracting effect of the diffraction structureS(x, y) (FIG. 7) in the diffraction plane 20. The relief structure R(x,y) (FIG. 4) is the diffraction grating 32 (FIG. 4) with a for examplesinusoidal profile and a spatial frequency f of less than 2400 lines/mm.The grating vector of the relief structure R(x, y) is in the diffractionplane 20. The superimposition function M(x, y) in the surface portion 13(FIG. 2), 14 (FIG. 2) and 15 (FIG. 2) of the security feature 16 isdetermined by the effect of the diffraction structure S(x, y), whereinthe light 11 which is incident on the layer composite 1, at apredetermined viewing angle +∂ and −∂ respectively, is deflected intothe positive diffraction order 23 (FIG. 3) or into the negativediffraction order 24 (FIG. 3) respectively. In the diffraction plane 20first beams 44 of the wavelength λ₁ include the viewing angle ∂ with theincident light 11 and second beams 45 of the wavelength 2 include theviewing angle −∂. The observer 35 (FIG. 5) perceives the surface portion13, 14, 15 at the viewing angle ∂ in the color of the wavelength λ.After rotation of the layer composite 1 in the plane thereof through180° the surface portion 13, 14, 15 appears to the observer 35 at theviewing angle −∂ in the color of the wavelength λ₂. If the centralsurface 33 involves the local inclination γ=0° the wavelengths λ₁ and λ₂do not differ. For other values of the local inclination γ thewavelengths λ₁ and λ₂ differ. The normal 21′ to the inclined centralsurface 33, shown in broken line, includes the angle α with the incidentbeam 11, wherein α=−β=γ. The first beams 44 and the normal 21′ includethe diffraction angle ξ₁, while the second beams 45 and the normal 21′include the diffraction angle ξ₂.

Because of ξ_(k)=asin(sin α+m_(k)·λ_(k)·f) and α=γ, the relationship forthe first two diffraction orders 23, 24, that is to say for m_(k)=±1, isas follows:f·(λ₁+λ₂)=2·sin(∂)·cos(γ)  (1),from which it follows that, for predetermined values of the viewingangle ∂ and the spatial frequency f, the sum of the two wavelengths λ₁,λ₂ of the beams 44, 45 is proportional to the cosine of the local angleof inclination γ. The equation (1) is to be easily derived for otherorder numbers m. The order numbers m and the viewing angle ∂ for a givenobservable color are determined by the spatial frequency f.

FIGS. 10 a and 10 b show by way of example an embodiment of the securityfeature 16, wherein in FIG. 10 a the security element 2 is rotatedthrough 1800 with respect to the security element 2 in FIG. 10 b, in theplane thereof. The diffraction plane 20 (FIG. 9) is illustrated by theline 36 thereof. In FIGS. 10 a and 10 b the security feature 16 includesthe three surface portions 13, 14, 15 with the diffraction structureS(x, y)=R(x, y)+M(x, y), wherein, in the three surface portions 13, 14,15, the diffraction structures S(x, y) differ by virtue of the values,determined by means of equation (1), in respect of the localinclinations y of the superimposition function M(x, y) and the spatialfrequency f of the relief profiles R(x, y). A background field 46adjoins at least one surface portion 13, 14, 15 and has the diffractiongrating 32 (FIG. 4) with the same relief profile R(x, y) and the spatialfrequency f which is specific to the background field 46. The gratingvector of the relief profile R(x, y) is oriented in parallelrelationship with the line 36 in the surface portions 13, 14, 15 and inthe background field 46. Upon perpendicular illumination of the securityelement 2 with white light 11 (FIG. 9), the surface portions 13, 14, 15and the background field 46 light in the same color in the securityelement 16 in the orientation shown in FIG. 10 a, at the viewing angle+∂, and the security feature 16 appears to light up without contrast ina uniform color for the observer 35 (FIG. 5), for example the deflectedfirst beams 44 (FIG. 9) are of the wavelength λ₁ for example 680 nm(red). In the orientation shown in FIG. 10 b, the entire securityfeature 16 is observed at the viewing angle −∂. For example the firstsurface portion 13 lights up in the second beams 45 (FIG. 9) of thewavelength λ₂, for example λ₂=570 nm (yellow), the second surfaceportion 14 lights up in the second beams 45 of the wavelength 3, forexample λ₃=510 nm (green) and the third surface portion 15 lights up inthe second beams 45 of the wavelength 4, for example λ₄=400 nm (blue).In the background field 46 in which the central surface 33 (FIG. 9) ofthe diffraction grating 32 (FIG. 4) involves the inclination y (FIG. 9)with the value γ=0, for symmetry reasons the second beams 45 are also ofthe wavelength λ₁, that is to say, the background surface 46 again emitsin the red color. The advantage of this embodiment is the strikingoptical characteristic of the security feature 16, namely the colorcontrast which is visible at a single predetermined orientation of thesecurity element 2 and which changes or disappears after a 180° rotationof the security element 2 about the surface normal 21 (FIG. 3). Thesecurity feature 16 thus serves to establish a predetermined orientationof the security element 2 with the security feature 16 which cannot beholographically copied.

It is only for the sake of simplicity that a uniform color, that is tosay a constant inclination γ, has been assumed to apply by way ofexample in each surface portion 13, 14, 15. In general terms the surfaceportion 13, 14, 15 has a portion from the superimposition function M(x,y) so that the inclination γ in the surface portion 13, 14, 15continuously changes in a predetermined direction and the wavelengths ofthe second beams 45 originate from a region on both sides of thewavelength λ_(k). Instead of the similarly delimited surface portions13, 14, 15 a plurality of the surface portions 13, 14, 15 arranged onthe background field 46 form a logo, a text and so forth.

In FIG. 11 the diffraction structure S(x, y) is of a more complicatednature. The superimposition function M(x, y) is a symmetrical,portion-wise steady, periodic function, the value of which varies alongthe co-ordinate axis x in accordance with z=M(x, y) while M(x, y) is ofa constant value z along the co-ordinate axis y. The for examplerectangular surface portion 13, 14 (FIG. 10), 15 (FIG. 10) is orientedwith its longitudinal side in parallel relationship with the co-ordinatex and is subdivided into narrow partial surfaces 47 of the width b, thelongitudinal sides of which are oriented parallel to the co-ordinateaxis y. Each period 1/F_(x) of the superimposition structure M(x; y)extends over a number t of the partial surfaces 47, for example thenumber t is in the range of values of between 5 and 10. The width bshould not be less than 10 μm as otherwise the diffraction structureS(x, y) is too little defined on the partial surface 47.

The diffraction structures X(x, y) of the adjacent partial surfaces 47differ in the summands, the relief profile R(x, y) and the portion ofthe superimposition function M(x, y), which is associated with thepartial surface 47. The relief profile R_(i)(x, y) of the i-th partialsurface 47 differs from the two relief profiles R_(i+1)(x, y) andR_(i−1)(x, y) of the adjacent partial surfaces 47 by at least onegrating parameter such as azimuth, spatial frequency, profile height h(FIG. 4) and so forth. If the spatial frequency F_(x) and F_(y)respectively are at most 10 lines/mm but not less than 2.5 lines/mm, theobserver 35 (FIG. 5) can no longer perceive any subdivision on thesurface portion 13, 14, 15 by the periods of the superimpositionfunction M(x, y), with the naked eye. Subdivision and occupation of thepartial surfaces 47 with the diffraction structure S(x, y) is repeatedin each period of the superimposition function M(x, y). In anotherembodiment of the security feature 16 the relief profile R(x, y) changescontinuously as a function of the phase angle of the periodicsuperimposition function M(x, y).

The diffraction structures S(x, y) shown in FIG. 11 are used in theembodiment of the security feature 16 shown in FIG. 12, which deploys anovel optical effect upon illumination with white light 11 when thesecurity feature 16 is tilted about the tilt axis 41 parallel to theco-ordinate axis y. The security feature 16 includes the triangularfirst surface portion 14 which is arranged in the rectangular secondsurface portion 13. In the first surface portion 14 the diffractionstructure S(x, y) is distinguished in that the spatial frequency f ofthe relief profile R(x, y) changes in the direction of the co-ordinateaxis x within each period of the superimposition function M(x, y)stepwise or continuously in a predetermined spatial frequency range δf,wherein the spatial frequency f_(i) is greater in the i-th partialsurface 47 (FIG. 7) than the spatial frequency f_(i-1) in the preceding(i-1)-th partial surface 47. In each period therefore the first partialsurface 47 involves the spatial frequency f of the value f_(A). For thepartial surface 47 at the minimum of the period, the spatial frequencyf=f_(M) and for the partial surface 47 at the end of the period, thevalue of the spatial frequency f=f_(E), wherein f_(A)<f_(M)<f_(E),wherein δf=f_(E)−f_(A). In the second surface portion 13 the diffractionstructure S(x, y) is distinguished in that the spatial frequency f ofthe relief profile R(x, y) decreases stepwise or continuously in thedirection of the co-ordinate axis x within a period of thesuperimposition function M(x, y) from the one partial surface 47 to thenext. In an embodiment, as an example, the diffraction structure S**(x,y)=R(−x, y)+M(−x, y) of the second surface portion 13 is the diffractionstructure S(x, y) of the first surface portion 14, which is mirrored atthe plane defined by the co-ordinate axes y, z. The grating vectors andthe line 36 (FIG. 11) of the diffraction plane 20 (FIG. 9) are orientedin substantially parallel relationship with the tilt axis 41 in bothsurface portions 13, 14. The gradient 38 is substantially parallel tothe plane defined by the co-ordinate axes x and z.

In FIG. 12 a the security element 16 is in the x-y-plane defined by theco-ordinate axes x and y, wherein the viewing direction 39 (FIG. 5)forms a right angle with the co-ordinate axis x. In the case ofperpendicularly incident white light 11 (FIG. 1) the partial surfaces 47are illuminated in the region of the minima of the superimpositionfunction M(x,

-   -   y). As those partial surfaces 47, in both diffraction structures        S(x, y), S**(x, y), involve the same relief profile R(x, y) and        the same inclination γ≈0°, the light beams 34 (FIG. 5) which are        diffracted into the viewing direction 39 at the two surface        portions 13, 15 originate from the same range of the visible        spectrum, for example green, so that the color contrast on the        security feature 16 disappears between the first surface portion        14 and the second surface portion 13. When the security feature        16 is tilted about the tilt axis 41 the color contrast becomes        clearer with an increasing tilt angle, as is shown in FIG. 12 b.        When the security feature is tilted towards the left the color        of the first surface portion 14 is displaced in the direction of        red as the partial surfaces 47 (FIG. 11) with the relief        profiles R(x, y) in respect of which the spatial frequency f is        less than f_(M) become effective. The color of the second        surface portion 13 is displaced in the direction of blue as the        partial surfaces 47 in respect of which the spatial frequency f        of the relief profile R(x, y) is greater than f_(M) become        effective. In FIG. 12 c the security feature 16 is tilted from        the position shown in FIG. 12 a towards the right about the tilt        axis 41. The color contrast also appears markedly upon tilting        towards the right, but with interchanged colors. The color of        the first surface portion 14 is displaced in the direction of        blue as the partial surfaces 47 in respect of which the spatial        frequency f of the relief profile R(x, y) is greater than the        value f_(M) become effective while the color of the second        surface portion 13 is displaced in the direction of red as the        partial surfaces 47 (FIG. 11) in respect of which the spatial        frequency f of the relief profile R(x, y) of the diffraction        structure S**(x, y) decreases with respect to the value f_(M)        become effective.

In another embodiment of the diffraction structure S(x, y) in FIG. 11the relief profile R(x, y) in the partial surfaces 47 of each period1/F_(x) involves the same spatial frequency but the relief profile R(x,y) differs from one partial surface 47 to another by virtue of itsazimuth angle φ of the grating vector relative to the co-ordinate axisy. Within a period 1/F_(x) the azimuth angle φ changes stepwise orcontinuously for example in the range δφp=±40° with φ≈0° in the minimumof each period. The azimuth angle φ is selected in dependence on thelocal inclination γ (FIG. 5) of the central surface 33 (FIG. 5) from therange δφ in such a way that on the one hand the diffraction structureS(x, y) of the first surface portion 14 (FIG. 12 a) at all tilt anglesabout the tilt axis 41 (FIGS. 12 b and 12 c), emits diffracted lightbeams 34 (FIG. 5) of the color range which is predetermined by means ofthe spatial frequency f, for example from the green range, in theviewing direction 39 (FIG. 5), and on the other hand the second surfaceportion 13 (12 a) in which the mirrored diffraction structure S**(x, y)is shaped lights up only at a single predetermined tilt angle in thepredetermined color, for example in a mixed color produced from thegreen range. At other tilt angles the second surface portion 13 is darkgray. For the azimuth angle range δφ±20° which is set forth here by wayof example, the green range extends from the wavelength λ=530 nm (φ≈0°)to the wavelength λ=564 nm.

In FIG. 13 the superimposition function M(x, y) used in the diffractionstructure S(x, y) is an asymmetrical function in the direction of theco-ordinate axis x. The superimposition function M(x, y) rises withinthe period 1/F_(x) aperiodically from a minimum value to a maximumvalue, for example like the function y=const·x^(1.5). The spatialfrequency F_(x) and F_(y) respectively is in the range of 2.5 lines/mmup to and including 10 lines/mm. Not shown herein are the discontinuitylocations which occur due to the operation modulo value H (FIG. 7). Theabove-described ‘anisotropic’ matt structure with the preferreddirection substantially parallel to the co-ordinate axis x is used asthe relief profile R(x, y). The incident light 11 (FIG. 5) is thereforescattered fanned out primarily parallel to the co-ordinate axis y. Thediffraction structure S(x, y)=R(x, y)+M(x, y) is shaped in the firstsurface portion 14 (FIG. 12 a) and the diffraction structure S**(x,y)=R(−x, y)+M(−x, y) is shaped in the second surface portion 13 (FIG. 12a). The optical effect of the security element 16 will be described withreference to FIG. 12 a, with light 11 (FIG. 9) incident on thex-y-plane. When the security element 16 is in the x-y-plane, theincident light 11 of great intensity is scattered by the matt structurein the region of the minima of the superimposition function M(x, y),while the scatter effect of the other surface portions 47 of thediffraction structures S(x, y), S**(x, y) is to be disregarded. Thelight which is backscattered by the surface portions 13, 14 involves thecolor of the incident light 11 (FIG. 5) and is of the same surfacebrightness in both surface portions 13, 14 so that it is not possible tosee any contrast between the two surface portions 13, 14. In FIG. 12 bthe incident light 11 (FIG. 5) is incident at an angle of incidence α onthe security element 16 which is tilted about the tilt axis 41 towardsthe left. The incident light 11 (FIG. 5) is only still scattered in thesecond surface portion 13. Under that illumination condition, thesurface brightness of the first surface portion 14 is orders ofmagnitude less than in the second surface portion 13 so that the firstsurface portion 14 stands out as a dark surface against the light secondsurface portion 13. In FIG. 12 c the security feature 16 is tilted awaytowards the right, in which case now the surface brightnesses of the twosurface portions 13 and 14 are interchanged.

In FIGS. 12 a through 12 c, instead of a single triangular first surfaceportion 14, it would be possible to arrange on the second surfaceportion 13 a plurality of the first surface portions 14 which form alogo, a text and so forth.

A further embodiment, instead of the simple mathematical functions, alsouses relief images as are employed on coins and medals, as an at leastportion-wise steady superimposition function M(x, y) in the diffractionstructure S(x, y), wherein the relief profile R(x, y) is advantageouslyan ‘isotropic’ matt structure. In this embodiment the observer of thesecurity element 2 has the impression of a three-dimensional image witha characteristic surface structure. When the security element 2 isrotated and tilted the distribution of brightness in the image changesaccording to the expectation in relation to a true relief image, butprojecting elements do not cast any shadow.

Without departing from the idea of the invention, all diffractionstructures S are restricted in respect of their structure height to thevalue H_(St) (FIG. 1), as was described with reference to FIG. 7. Therelief profiles R(x, y) and superimposition functions M(x, y) used inthe above-described specific embodiments can be combined as desired toafford other diffraction structures S(x, y).

The use of the above-described security features 16 in the securityelement 2 has the advantage that the security feature 16 forms aneffective barrier against attempts to holographically copy the securityelement 2. In a holographic copy the positional displacements or colorshifts on the surface of the security element 16 are only to beperceived in an altered form.

1. A security element comprising a layer composite with microscopically fine optically effective structures of a surface pattern, which are embedded between transparent layers of the layer composite, wherein the optically effective structures are shaped into a reflecting interface between the layers in surface portions of a security feature in a plane of the surface pattern, which is defined by co-ordinate axes (x; y), wherein at least one surface portion of dimensions greater than 0.4 mm has comprises a diffraction structure formed by additive or substractive superimposition of a superimposition function (M) describing a macroscopic structure, with a microscopically fine relief profile (R), wherein the superimposition function (M), the relief profile (R) and the diffraction structure are functions of the co-ordinates (x; y) and the relief profile (R) describes a light-diffracting or light-scattering, optically effective structure which, following the superimposition function (M), retains the predetermined relief profile (R), and wherein a central surface defined by the at least portion-wise steady superimposition function (M) is curved at least in partial regions and at any point has a local angle of inclination predetermined by the gradient of the superimposition function (M), is not a periodic triangular or rectangular function and changes slowly in comparison with the relief profile (R).
 2. A security element as set forth in claim 1, wherein the superimposition function (M) is a portion-wise steady, periodic function with a spatial frequency of at most 20 lines/mm.
 3. A security element as set forth in claim 1, wherein the superimposition function (M) is an asymmetrical, portion-wise steady, periodic function with a spatial frequency in the range of between 2.5 lines/mm and 10 lines/mm.
 4. A security element as set forth in claim 1, wherein adjacent extreme values of the superimposition function (M) in the surface portion are remote from each other by at least 0.025 mm.
 5. A security element as set forth in claim 2, wherein relief profile (R) is a diffraction grating of constant profile height, which has a grating vector with an azimuth angle and with a spatial frequency of greater than 300 lines/mm.
 6. A security element as set forth in claim 2, wherein the relief profile (R) is an anisotropic matt structure which has a preferred direction with an azimuth angle.
 7. A security element as set forth in claim 5, wherein the security feature has at least two adjacent surface portions and wherein the first diffraction structure is shaped in the first surface portion and the second diffraction structure which differs from the first diffraction structure is shaped in the second surface portion, wherein the grating vector or the preferred direction of the first relief profile (R) in the first surface portion and the grating vector or the preferred direction of the second relief profile (R) in the second surface portion are directed substantially parallel.
 8. A security element as set forth in claim 5, wherein the diffraction structure the grating vector or the preferred direction of the relief profile (R) is substantially parallel to a gradient plane which is determined by the gradient of the superimposition function (M) and a surface normal which is perpendicular to the surface of the layer composite.
 9. A security element as set forth in claim 5, wherein shaped in a first surface portion is the first diffraction structure which is formed as the sum of the relief profile (R) and the superimposition function (M) and wherein shaped in a second surface portion is the second diffraction structure which is formed as the difference (R−M) of the same relief profile (R) and the same superimposition function (M).
 10. A security element as set forth in claim 5, wherein in the diffraction structure the grating vector or the preferred direction of the relief profile (R) is substantially perpendicular to a gradient plane which is determined by the gradient of the superimposition function (M) and a surface normal which is perpendicular to the surface of the layer composite.
 11. A security element as set forth in claim 3, wherein the relief profile (R) is a diffraction grating which has a grating vector with an azimuth angle and a spatial frequency greater than 300 lines/mm, wherein the surface portion in each period (1/F) of the superimposition function (M) is subdivided into a number t of partial surfaces of the width 1/(F·t), wherein F is a spatial frequency of the superimposition function (M), wherein that the diffraction grating of the diffraction structure, which is associated with the one partial surface, differs in at least one of the grating parameters from the diffraction gratings of the adjacent partial surfaces, wherein that the subdivision and the occupation of the partial surfaces with the diffraction structure is repeated in each period (1/F) of the superimposition function (M) and wherein the diffraction grating has the azimuth angle and/or the spatial frequency corresponding to the local inclination in the surface portion and wherein within each period (1/F) the grating parameters of the diffraction grating step-wise or continuously traverse a predetermined azimuth angle range or a predetermined spatial frequency range respectively.
 12. A security element as set forth in claim 5, wherein in the first surface portion the first diffraction structure is formed from the sum of the relief profile (R) and the superimposition function (M) and wherein in the second surface portion the second diffraction structure is formed from the first diffraction structure (S), the second diffraction structure being the first diffraction structure which is mirrored at the plane of the surface pattern.
 13. A security element as set forth in claim 5, wherein the diffraction structure formed as the sum of the superimposition function (M) and the relief profile (R) is shaped in at least one surface portion, wherein the spatial frequency of the relief profile (R) is less than 2400 lines/mm and the superimposition function (M) has a local inclination (γ) measured in the diffraction plane of the relief profile (R), wherein the surface portion adjoins a background field of the security feature, wherein the background field parallel to the cover layer has the central surface with the local inclination γ=0° into which a sinusoidal diffraction grating with a second spatial frequency and with a grating vector oriented in parallel in the diffraction plane of the relief profile (R) is shaped, wherein the second spatial frequency is so selected that upon perpendicular illumination with white light in one viewing direction at a predetermined positive viewing angle the surface portion and the background field do not differ with respect to of the color of the diffracted light and wherein that after a 180° rotation of the layer composite about the surface normal at the negative viewing angle the surface portion and the background field differ with respect to the color of the diffracted light.
 14. A security element as set forth in claim 1, wherein the relief profile (R) is an isotropic matt structure.
 15. A security element as set forth in claim 14, wherein the superimposition function (M) describes a relief image.
 16. A security element as set forth in claim 14, wherein the superimposition function (M) describes a portion of a sphere.
 17. A security element as set forth in claim 1, wherein the diffraction structure is restricted to a structure height of less than 40 μm and the superimposition function (M) is restricted to a variation value (H) of less than 30 μm, wherein the value of the superimposition function (M), which is used in the diffraction structure is equal to {(M)+C(x; y)} modulo variation value (H)−C(x; y), wherein the function C(x; y) is restricted in amount to half the structure height.
 18. A security element as set forth in claim 1, wherein surface elements having optically effective structures are parts of the surface pattern and at least one of the structure elements adjoins the security feature.
 19. A security element as set forth in claim 1, wherein arranged on at least one of the surface portions is at least one identification mark with an optically effective structure differing from the diffraction structure and, wherein that identification mark which can be used as a reference for orientation of the layer composite comprises an optically effective structure comprising at least one of a diffractive relief structure, a light-scattering relief structure and a mirror surface. 